A randomised observer masked clinical study was conducted to assess the additive effect of latanoprost (13,14-dihydro-17-phenyl-18,19,20-trinorprostaglandin F2 alpha-isopropylester) to timolol maleate in patients with elevated intraocular pressure (IOP). Patients were randomly assigned to two treatment groups. One group (n = 10) received timolol, the other group (n = 9) received latanoprost twice daily for 1 week. After 1 week all patients received both timolol and latanoprost. Eyes treated with timolol (mean diurnal IOP (SD) day 0, 24.2 (2.8) mm Hg) and latanoprost (mean diurnal IOP day 0, 28.5 (5.6) mm Hg) showed an IOP reduction of 5.9 (2.3) mm Hg (24%) and 8.9 (2.5) mm Hg (31%), respectively after the first week. Adding latanoprost to the eyes treated with timolol as well as timolol to the eyes receiving latanoprost gave a further reduction of 2.6 (1.1) mm Hg (13%) and 2.6 (2.2) mm Hg (14%), respectively. Only mild transient hyperaemia was observed in patients receiving latanoprost. The results indicate that latanoprost and timolol can be combined successfully and that complete or almost complete additivity is reached even at pressure levels below 20 mm Hg.